In all radio frequency (RF) communication systems, the power needed to transmit a signal consumes a large proportion of the available energy. An increase in the efficiency of the transmitting circuit of a communication device would decrease its power consumption. In addition, low power consumption due to higher efficiency reduces the heat generated in each communication device due to internal power dissipation.
Compact size and low weight are desirable features in cellular and other portable radio telephones (hereinafter referred to as mobile units). The power needed to operate a mobile unit is usually drawn from a battery. Since the battery is typically the largest and heaviest device in a mobile unit, increasing the efficiency of the mobile unit--and specifically of the transmitting circuit--would allow a smaller battery to be used, thereby decreasing the overall size and weight of the mobile unit.
The transmitting circuit of a typical mobile unit includes an RF source, a driver amplifier circuit, a power amplifier circuit and an antenna. (The driver amplifier and the power amplifier are sometimes referred to together as a two-stage power amplifier circuit.) The driver amplifier circuit and the power amplifier circuit include power transistors which amplify an RF signal generated by the RF source.
When the typical mobile unit is active, the driver and power amplifier are continuously on. Applicants have recognized, however, that the power needed to transmit a signal from the mobile unit to a base station does not always require the full operating power of the driver and power amplifiers. That is, it is estimated that for many applications the power amplifier is actually only required approximately twenty percent of the time (e.g., near the fringe of a cell, behind a structure, etc.) Accordingly, during the majority of the time the mobile unit is being used, the driver amplifier alone can produce a signal strong enough so that it will be detected by the base station.
Presently, several techniques have been devised that attempt to increase the efficiency of power amplifiers. These techniques include circuits that control the drain or collector supply voltage of the transistors used in a linear power amplifier; utilization of quasi-linear amplifiers; and utilization of non-linear precompensation circuits (e.g., a feedback controlled predistortion circuit). However, each of the aforementioned techniques require additional--and usually complex--circuitry and have difficulty maintaining the desired linear amplification (this is especially true because of the harsher environmental conditions to which mobile units are exposed). Furthermore, these techniques usually increase the heat generated in the mobile unit due to internal power dissipation of the additional circuitry.